I've had a letter through to get my flu jab(I have over a decade to go until I am 65), as has my son aged 15.
My understanding is that whilst over 50s are getting the flu jab this year they can only access it later whereas the normal qualifying vulnerable groups should be able to access it now ie over 65s, asthma and carers.
It might be a misunderstanding of the rules by the GPs receptionist.
Whilst there are a lot of colds/viruses in circulation in early winter which are similar to mild flu, flu isn't normally in circulation until mid December so provided you get a jab by the end of November you should be OK. (They determine if it is flu by lab testing - and other viruses don't lead to pneumonia in the same way as flu does).
I really don't understand anyone not getting the flu jab who is entitled to one, 2 years ago my daughter and husband got flu (she hadn't had the jab as she always gets a mild illness afterwards and we couldn't get an appointment which fitted in with races). Both were on the sofa and unable to do anything for 2 weeks, my husband hadn't had a day off sick for over a decade and cycled 10 miles to work each day, and my dau ended up with pneumonia. My dau was racing elite level cyclocross at the time with over half the women she was racing against being full time professionals - so she was very fit but had asthma which led to the pneumonia. The Dr commented her resting pulse was normal at 75 - when we pointed our her normal resting pulse was low 40s he changed his notes to abnormal.
If you are entitled to the jab please get it.